1 68 DISTRIBUTION OF GAS THROUGH MAINS. 



was expected, the gas must be forced through the leading main at a greater 

 pressure. 



In this example, if the diameter of the main for the present consumption 

 be twelve inches, but if to secure an adequate supply at any future period its 

 diameter should be increased to fifteen inches, the present working pressure 

 may be reduced to l^ths of an inch instead of three inches ; and as the 

 leakage will also be decreased, the extra-sized main will not be found dis- 

 advantageous even in the first instance. 



We will suppose again that the level of the district to be lighted varies in 

 no place more than thirty feet from the datum line ; in which case the whole 

 district may be supplied direct from the equilibrium cylinder, without the in- 

 terposition of a governor. The quantity of gas required for each separate 

 street must now be ascertained, and the proper-sized pipes (which will be 

 found in the Tables) be taken from the cylinder down each main street, the 

 pressure being considered equal to five-tenths of an inch. The cross streets 

 will be supplied from the main streets, and the sum of all the quantities and 

 the several distances from one another will give the various diameters of the 

 pipes. 



The direction in which the town will be improved or extended is generally 

 pretty well known, therefore there can be no difficulty in arranging the dis- 

 charge mains so as to meet the extra supply in that quarter. Every main 

 and cross pipe, whatever may be their diameters, must be connected together 

 in every available place, and thus form a system of mains. On no other plan 

 can a certain and regular pressure be ensured ; deficiencies in the quantity of 

 gas in one place will be made up by a supply from another point, in which 

 there may be an excess, and thus cause a constant circulation. The pressure 

 in the mains will vary directly as the rise above or the fall below the datura 

 line, at the constant rate of one-tenth of an inch for every ten feet. Thus, 

 at those points which rise ten feet above the datum the pressure will be 

 increased one-tenth, and will be decreased one-tenth at a point ten feet below 

 the datum. If, therefore, these points are connected together, the discharges 

 will be equalized ; and so at every intermediate elevation. 



The various inconveniences arising from the scarcity of gas in some divi- 

 sions, and an excess in others, are often severely felt, sometimes to such an 

 extent that the atmospheric air will take the place of the gas, and cause what 

 is termed a " blow." An excess of pressure causes a wasteful expenditure of 



